Global ChangeIntersection of Nature and Culture

If we switch to biofuels, how much do indirect greenhouse gas emissions matter?

Saturday, December 5th, 2009

In this week’s issue of Science, Jerry Melillo and colleagues investigate¹ what kinds of impacts might arise from changing land use to grow more cellulosic biofuel crops.  If you think about it, the switch to biofuels could have a big impact on greenhouse gas emissions—and not in a good way.  For instance, clearing a forest or pastureland to grow a biofuel crop could cause a net release of carbon from the ecosystem, as plant growth changes, biomass is lost, and soil decomposition increases.

Using a model of the world economy coupled to a terrestrial ecosystem model, they considered two cases:

• Case 1:  Natural lands (e.g., forests and pastures) are allowed to be converted to meet increasing biofuel demand.
• Case 2: Existing managed lands are managed even more intensely to generate biofuel demand.

What did they find?

• Both cases generated a global biofuel crop area exceeding the current agricultural area used to produce food.  Not surprisingly, the two cases generated different land use outcomes.  Case 1 led to more loss of forests and pastures than case 2.
• In Case 1, there is a much larger overall loss of carbon from ecosystems caused by land use change.  Less carbon was released in case 2 because of lower deforestation and increased use of pasture/shrubland/savanna—all of which can begin to accumulate carbon in soils when managed for biofuels.
• The loss of nitrogen from fertilizer becomes a larger source of greenhouse gas emissions (as N₂O) than land use change.  Although you might expect more fertilizer use and greater greenhouse gas loss with more intensively in case 2, this wasn’t the case, with both using roughly the same amount.
• Overall, how do biofuels fare?  Case 2 ends up releasing fewer greenhouse gases overall, not surprisingly because there is less land conversion.  However, they found that you need to consider the benefits of biofuel production for a long time period (2009-2100) rather than shorter time (2009-2030) because the large initial carbon releases caused by land use and changes and nitrogen releases from fertilizer actually make using biofuels worse than not using them over the short term.

¹ Melillo, J.M. et al. (2009) Indirect emissions from biofuels: How important? Science 326:1397-1399.

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